Wood is a great building material for the structure, but do you know what its not good for? Being quiet. Because of this I need to cover the top side of the foley pits, which I used canvas for. The poly-fil was chosen to act as an extra layer of padding for rocks and sticks being kicked around the pit.
I started by cutting out a square of poly-fil large enough to cover the floor of the pit and and 4 strips to cover the side walls. These by no means have to be exact. Just measure the inside dimensions of your pit and give the poly-fil a rough cut.
I then used a staple gun to fasten the poly-fil and canvas to the wood: 1/2 inch staples.
In terms of canvas I got a nice long amount (about 10 feet) and 5 feet wide. Because we had two pits we cut the canvas in half, giving each pit a 5 x 5 section of, ensuring the entirety of the pit and sides were covered.
I started with the pits flipped right-side-up, laid the canvas over them and pushed in at the sides making sure to keep the center of the canvas taught. I put a staple in each corner and half-way between corners. Continue up the side of the pits and staple the canvas to the 2-by-4s.
With the inside of your foley pit upholstered, flip it over so the flat wooden walking surface is on top. It is important to keep this surface uncovered by canvas as it makes a great hard-wood footstep sound.
Working one at a time, fold the ends of the canvas in on each other until you have the perfect amount of canvas to cover the outside portion of the 2-by-4s without creeping up onto the flat wood walking surface.
Folding the canvas here gives the foley pit a nice clean look, as opposed to just cutting the canvas and having its ends eventually fray.